homemade EFIS system and EMI

jcpearce wrote:
As a pet/learning project I made a data aquisition unit using an 8051
microprocessor and an EPIA M motherboard running a variant of Linux
to
process and display the information. It all works but the EMI from
the
EPIA M causes way too much noise to the aircraft radios. I have tried
shielding the whole device in an aluminum case with very little
improvement.

Any ideas on how to smother the EMI or some other small motherboard
which may not have as much an issue (as a test I took my portable
aviation radio and within 6' of any my home computers the same occurs
which gives me little hope)

Thanks

Hi, JC!
As others have mentioned, it is important that the conductors carrying
signal and power in and out of the case be prevented from carrying the
RFI/EMI out of the box; ferrite beads and high-quality ceramic caps or
filter connectors can take care of that problem. Try not to use too
high a value of cap as the self-resonant frequency decreases with size.
Usually around 100pf is good at VHF. Your circuits have many frquency
dividers which reduce the initial oscillator frequency down to much
lower values. These square-waves are rich in harmonics. If you tune an
FM receiver or a VHF receiver across each band you can recognize the
various harmonics by seeing where the amplitude peaks. You can make a
list of these and see the periodicity of the responsible waveform. The
rf energy inside your box induces currents in the metallic structure of
the box. Any gaps, no matter how small, form a slot antenna which
radiates. Each fastener around the periphery of the box where the sides
and top and bottom attach when tightened cause a slight arch in the
surfaces which form the slots. Think of the pan and rocker-arm covers
opn a car engine! There are several ways to stop this. Use conductive
elastomer gaskets at each joint, make the box out of extremely thick
material with machined mating surfaces with joggles, or cover the box
with rf absorbent material with an overall metal cover such as aluminum
foil. Any rf energy leaking out of the box gaps will be absorbed
somewhat as they pass through the absorber, will reradite from the foil
back through the material, and then through many passes back and forth
be attenuated. One of the manufacturers of rf gaskets has an excellent
description of the radiation from a box in their product manual. Paul